1. The comment you're talking about is from, essentially, someone who doesn't know what they're talking about. This isn't really meaningfully a "game changer" in terms of "Should I buy an iPhone?"
2. Computers are already used to analyze many of these types of diagnostic tests. It may have been easier to port those systems to a smart phone than to develop a new, bespoke microcontroller. Expertise is a cost.
3. There are massive benefits to having this hooked up to a smart phone, and as you have mentioned, much of it goes to extra functionality. During the Ebola outbreak, case reports often had to be dealt with in paper forms processed by hand, which slowed both reporting and contact tracing. From a public health perspective, being able to link test results (deidentified or not) with a central repository at a ministry of health is a big deal.
Imagine the use case of this not as one person and a smart phone testing themselves. Imagine it as one person and a smart phone testing 4 villages over the course of the week, in conjunction with actually getting people treatment.